1. Field of the Invention
The present invention relates to compositions of tannin containing polymers, and in particular relates to methods of using the same for conditioning microbial mixed liquor and improving flux in membrane bioreactor (MBR) systems.
2. Description of Related Art
Biological treatment of wastewater for removal of dissolved organics is well known and is widely practiced in both municipal and industrial plants. This biological process is generally known as the “activated sludge” process in which micro-organisms consume organic compounds through their growth. The process necessarily includes sedimentation of the micro-organisms or “biomass” to separate it from the water and complete the process of reducing Biological Oxygen Demand (BOD) and TSS (Total Suspended Solids) in the final effluent. The sedimentation step is typically done in a clarifier unit. Thus, the biological process is constrained by the need to produce biomass that has good settling properties. These conditions are especially difficult to maintain during intermittent periods of high organic loading and the appearance of contaminants that are toxic to the biomass.
Typically, an activated sludge treatment has a conversion ratio of organic materials to sludge of up to about 0.5 kg sludge/kg COD (chemical oxygen demand), thereby resulting in the generation of a considerable amount of excess sludge that must be disposed of. The expense for the excess sludge treatment has been estimated at 40 to 60 percent of the total expense of a wastewater treatment plant. Moreover, a conventional disposal method of landfilling sludge may cause secondary pollution problems. Therefore, interest in methods to reduce the volume and mass of excess sludge has been growing rapidly.
Membranes coupled with biological reactors for the treatment of wastewater are well known, but are not widely practiced. In these systems, ultrafiltration (UF), microfiltration (MF), or nanofiltration (NF) membranes replace sedimentation of biomass for solids-liquid separation. A membrane can be installed in a bioreactor tank or in an adjacent tank where mixed liquor is continuously pumped from the bioreactor tank and back produces effluent with much lower total suspended solids (TSS), typically less than 5 mg/L, compared to 20 to 50 mg/L from a clarifier.
More importantly, membrane biological reactors (MBR) de-couple the biological process from the need to settle the biomass, since the membrane sieves the biomass from the water. This allows operation of the biological process at conditions that would be untenable in a conventional system including: (1) high mixed liquor suspended solids (bacteria loading) of 10 to 30 g/L, (2) extended sludge retention time, and 3) short hydraulic retention time. In a conventional system, such conditions may lead to sludge bulking and poor settleability.
The benefits of an MBR operation include low sludge production, complete solids removal from the effluent, effluent disinfection, combined COD, solids and nutrient removal in a single unit, high loading rate capability, and minimal problems with sludge bulking. Disadvantages include aeration limitations, membrane fouling, and membrane costs.
Membrane fouling can be attributed to surface deposition of suspended or dissolved substances. An MBR membrane interfaces with the biomass which contains aggregates of bacteria or “flocs”, free bacteria, protozoan, and various dissolved microbial products (SMP). The term SMP has been adopted to define the organic compounds that are released into the bulk microbial mixed liquor from substrate metabolism (usually biomass growth) and biomass decay.
In operation, the colloidal solids and SMP have the potential of depositing on the surface of the membrane. Colloidal particles form layers on the surface of the membrane, called a “cake layer.” MBR processes are designed to use rising coarse air bubbles to provide a turbulent cross flow velocity over the surface of the membrane. This process helps to maintain the flux through the membrane, by reducing the build up of a cake layer at the membrane surface.
Compared to a conventional activated sludge process, floc (particle) size is reportedly much smaller in typical MBR units. Small particles can plug the membrane pores, a fouling condition that may not be reversible. Since MBR membrane pore size varies from about 0.04 to about 0.4 micrometers, particles smaller than this can cause pore plugging. Pore plugging increases membrane resistance and decreases membrane flux.
Efficient and stable operation of MBR systems largely depends on the conditions and qualities of the biological populations of the biomass in the MBR system. The characteristics of the mixed liquor, including viscosity, extracellular polymeric substances (EPS), floc size, and colloidal and soluble organic substances, affect membrane filterability. While traditional approaches mostly rely on optimization of hydrodynamics and air scouring to reduce membrane fouling in MBR systems, new efforts are more devoted to coagulate and flocculate the activated sludge by adding chemicals and thereby to bind colloids and other mixed liquor components in flocs. These filterability enhancement chemicals can not only have a positive impact to decrease soluble foulants in the bulk phase and also improve the hydraulic permeability of the cake formed on the surface of the membrane.
Recently increasing efforts have been devoted to improve microbial mixed liquor filterability and enhance membrane flux in MBR systems. Options include use of inorganic coagulants such as ferric and aluminum salts and aluminum polymers, powdered activated carbon (PAC) and other type of inert particles (e.g. resins), and water soluble polymers. Use of inorganic coagulants will increase sludge generation and are only applicable to a narrow pH range. Addition of powdered activated carbon to MBR systems will not only increase sludge concentration, it may also cause irreversible permeability loss due to membrane pore plugging by PAC, and membrane wear due to the abrasiveness of the PAC. These problems will exaggerate and additional fouling may develop when the added PAC concentration becomes higher (e.g. 600 mg/L or above).
Various patents disclose the use of water-soluble polymers in MBRs for biomass conditioning and membrane flux enhancement. U.S. Pat. No. 6,723,245 discloses a method of using water soluble cationic polymers in MBRs. U.S. Pat. No. 6,872,312 discloses a method of using high molecular weight water soluble polymers in MBR systems. U.S. Pat. No. 6,926,832 discloses a method of using water soluble polymers in an MBR. U.S. Publication No. 2004/0168980 discloses a combination polymer treatment for flux enhancement in a MBR. U.S. Publication 2006/0272198 discloses a method for improving flux in an MBR. U.S. Pat. No. 7,378,023 discloses a method of using cationic polymers having a molecular weight greater than about 200,000 in an MBR for industrial wastewater treatment. The polymers in U.S. Pat. Nos. 6,723,245, 6,872,312, 6,926,832 and 7,378,023 and U.S. Publication Nos. 2004/0168980 and 2006/0272198 include polymers of (meth)acrylamide and one or more cationic monomers, cationic polymers having a molecular weight greater than about 200,000 Daltons, copolymer of acrylamide and one or more cationic monomers, and other polyamine coagulants. They do not claim tannin-containing copolymers.
U.S. Pat. Nos. 4,558,080, 4,734,216 and 4,781,839 disclose a tannin based polymer obtained by reacting tannin with an amino compound and an aldehyde under acidic conditions for use as a flocculant. The manufacturing process requires careful monitoring of the pH and intermediate viscosity during the reaction to prevent the batch from gelling. The long-term stability of the product and the amount of residual amine and formaldehyde in the solution may cause handling concerns.
U.S. Pat. No. 4,558,080 discloses the production of stable tannin-based flocculants made by polymerizing tannin with an aldehyde such as formaldehyde and an amino compound, such as monoethanolamine, while monitoring the viscosity of the reacting mixture. U.S. Pat. No. 4,734,216 discloses a flocculating compound comprised of polymerized tannin described in the above referenced patent in combination with an inorganic flocculant such as aluminum sulfate or iron chloride. U.S. Pat. No. 5,643,462 discloses a composition comprised of a water soluble/dispersible tannin containing polymer obtained by polymerizing ethylenically unsaturated monomers with tannin, the method of preparing the same and their use for water clarification. U.S. Pat. No. 6,478,986 teaches a process for the production of a quaternary tannate as a coagulating/flocculating agent, and its use for treating drinking water and water used in industry. The coagulating/flocculating agent is a vegetable polyelectrolytic cation. U.S. Pat. No. 4,990,270 discloses a thickening agent prepared by graft copolymerizing acrylamide and cationic monomer with water insoluble lignin in a calcium chloride/dimethylsulfoxide solution. The procedure is quite complicated and requires precipitation in acetone and filtration, and dialysis to isolate the product. The resulting material is used for enhanced oil recovery.
Accordingly, a need exists for time-saving and cost-effective polymer based chemicals for membrane flux enhancement, MBR efficiency improvement, and mixed liquor filterability enhancements.